1. Field of the Invention
The present invention relates to semiconductor pressure sensor devices and a method of forming the same, and more particularly, to a semiconductor pressure sensor device constituted by semiconductor pressure sensor chips having a pressure sensor for sensing pressure, an amplifying circuit for amplifying the signal from the pressure sensor, and electrodes integrated on a semiconductor substrate, and a lead frame where the semiconductor pressure sensor chip is mounted, and a method of manufacturing thereof.
2. Description of the Background Art
The method of directly die-bonding the semiconductor pressure sensor chip to the lead frame is generally known in conventional semiconductor pressure sensor devices.
FIG. 12 is a plan view of a conventional semiconductor pressure sensor chip; FIG. 13 is a side view of the semiconductor pressure sensor chip of FIG. 12; and FIG. 14 is a sectional view taken along plane B/B of FIG. 12. Referring to FIGS. 12-14, a semiconductor pressure sensor chip comprises a pressure sensor 51 with an embedded gauge resistor for sensing pressure, an integrated circuit 52 having an amplifying circuit and the like for amplifying the signal from pressure sensor 51, electrodes 53 for external leading, and a diaphragm constitution 54 for forming a diaphragm of pressure sensor 51.
FIG. 15 is a plan view of a lead frame 200 where the semiconductor pressure sensor chip is mounted. FIG. 16 is a side view of lead frame 200 of FIG. 15. Referring to FIGS. 15 and 16, lead frame 200 comprises a diepad 20 which is the chip mounting section where the semiconductor pressure sensor chip is mounted, a pressure receiving opening 3 formed in diepad 20, inner leads 5, a diver 6 for preventing resin from flowing over, and outer leads 7.
FIG. 17 is a plan view of a completed semiconductor pressure sensor device where the conventional semiconductor pressure sensor chip 50 is mounted on lead frame 200. FIG. 18 is a side view of the semiconductor pressure sensor device of FIG. 17. Referring to FIGS. 17 and 18, semiconductor pressure sensor chip 50 is directly die-bonded to diepad 20 of lead frame 200. Electrode 53 of semiconductor pressure sensor chip 50 and inner lead 5 are wire bonded by a metal fine wire 15 FIG. 19 is an enlarged partial view of C--C of diepad 20 in the semiconductor pressure sensor device of FIG. 17. Referring to FIG. 19, diepad 20 and semiconductor pressure sensor chip 50 are fixed together by an adhesive 30.
It is mentioned above that semiconductor pressure sensor chip 50 is directly die-bonded to lead frame 200 by adhesive 30 in a conventional semiconductor pressure sensor device. The expansion coefficients of lead frame 200 and semiconductor pressure sensor chip 50 are different to that of adhesive 30 in a conventional semiconductor pressure sensor device. This causes the generation of a thermal distortion, leading to the inconvenience that tension and bending moment are exerted upon pressure sensor 51 of semiconductor pressure sensor chip 50.
FIG. 20 is a diagram for explaining the tension and bending moment generated in semiconductor pressure sensor chip 50 and adhesive 30. Referring to FIG. 20, tension F.sub.51 and bending moment M.sub.51 are generated by thermal distortion in pressure sensor 51 of semiconductor pressure sensor chip 50. Tension F.sub.30 and bending moment M.sub.30 are generated in adhesive 30. Tension F.sub.20 and bending moment M.sub.20 are generated in diepad 20. This gives the balanced state expressed by the following equations of (1) and (2) in the case of thermal distortion. EQU F.sub.51 +F.sub.20 +F.sub.30 =0 (1) EQU M.sub.51 +M.sub.20 +M.sub.30 =0 (2)
Therefore, tension F.sub.51 and bending moment M.sub.51 acting on pressure sensor 51 of semiconductor pressure sensor chip 50 are expressed as in the following equations (3) and (4). EQU F.sub.51 =-(F.sub.20 +F.sub.30) (3) EQU M.sub.51 =-(M.sub.20 +M.sub.30) (4)
There was also a disadvantage that the distribution of thermal stress caused by the difference in expansion coefficients is not in uniform because the symmetry of integrated circuit 52 and electrodes 53 with the other portions is not appropriate, when pressure sensor 51 is considered as the center in FIG. 19. This led to an inconvenience that measurement of high accuracy could not be achieved.
Therefore, a method of mounting the semiconductor pressure sensor chip on a lead frame with a silicon base formed of silicon monocrystal therebetween was considered to absorb and decrease the thermal stress exerted upon pressure sensor 51. This approach was taken due to the fact that the material and hence the expansion coefficient of the silicon base is equal to that of semiconductor pressure sensor chip 50, resulting in substantial suppression of thermal stress generation.
Regarding the method of mounting semiconductor pressure sensor chip 50 onto lead frame 200 with a silicon base formed of silicon monocrystal therebetween, firstly the semiconductor pressure sensor chip is die-bonded on the silicon base. Then this silicon base is die-bonded to lead frame 200, followed by wire bonding electrode 53 of semiconductor pressure sensor chip 50 to inner lead 5 of lead frame 200 by metal fine wire 15. However, this method of mounting the semiconductor pressure sensor chip 50 to lead frame 200 requires the implementation of a silicon base for absorbing and relaxing thermal stress. Die-bonding therefore must be carried out two times, complicating the manufacturing steps to increase the cost.
It is necessary to form a pressure measuring chamber (cavity) for measuring air pressure in the lead frame with the semiconductor pressure sensor chip 50 shown in FIGS. 17 and 18 mounted, when used as a fine differential pressure sensor for measuring air pressure. FIGS. 21A and 21B show sectional structures of a semiconductor pressure sensor device having a cavity for measuring air pressure. FIG. 21C is a plan view of the semiconductor pressure sensor device of FIGS. 21A and 21B without a cap. Referring to FIGS. 21A-21C, a semiconductor pressure sensor device in the case of measuring air pressure comprises a semiconductor pressure sensor chip 50 for measuring air pressure, a lead frame 200 having semiconductor pressure sensor chip 50 mounted, a base 70 having lead frame 200 mounted above, and a cap 80 attached to lead frame 200 over lead frame 200 and semiconductor pressure sensor chip 50. Two cavities for measuring air pressure are formed by base 70, cap 80, and diepad 20 provided in lead frame 200. Specifically, pressure inlets A and B are provided individually as shown in FIG. 21C. Pressure entering through pressure inlet A is introduced into cavities A.sub.1 and A.sub.2, as shown in FIG. 21A. Pressure entering pressure inlet B is introduced to cavities B.sub.1 and B.sub.2, as shown in FIG. 21B. By measuring pressure individually by two pressure sensor chips 50, the measuring accuracy is improved in comparison with measurement by only one pressure sensor chip 50. In a semiconductor pressure sensor device having such an structure, a method is employed of attaching base 70 and cap 80 to lead frame 200 where semiconductor pressure sensor chip 50 is mounted. However there was an inconvenience that the adhesive resin for attaching base 70 and cap 80 to lead frame 200 flows to the semiconductor pressure sensor chip 50 side to adhere to semiconductor pressure sensor chip 50. This aggravates the characteristics of semiconductor pressure sensor chip 50.
Diepad 20 of a conventional lead frame 200 is supported asymmetrical by the outer frame of lead frame 200 and one of the inner leads 5, as shown in FIG. 17. When lead frame 200 is attached to base 70, as shown in FIG. 21A, using an adhesive resin of high viscosity as the adhesive, there was the inconvenience that the portion not supported by the diepad was raised by the adhesive resin applied on the surface of base 70. This led to a problem that it was difficult to fix diepad 20 on base 70 horizontally. A fixing jig had to be used in attaching diepad 20 to base 70 for solving the above mentioned problems, resulting in the complication of the manufacturing process.
Because the two diepads 20 have an isolated structure, as shown in FIG. 17, there was also the problem that physical continuity could not be maintained regarding the deformation which the two semiconductor pressure sensor chips 50 receive if external force is exerted on lead frame 200. In the case where the semiconductor pressure sensor device of FIGS. 21A-21C is used as the fine differential pressure sensor, it was necessary to provide the two semiconductor pressure sensor chips 50 within the two sealed cavities, respectively, where the divided portion between the two diepads of FIG. 17 had to be sealed with an adhesive resin at a later step. However, the sealing performance could not be improved because there was difference in the thickness of the adhesive resin at the divided portion and the diepad 20 portion.